The invention relates to a load drive system which drives a load in a forward or reverse direction in response to a command issued in the form of a switch operation by an operator, and in partiuclar, to a control system for a safety load drive including an object sensor which detects the presence of an object standing in the way to the movement of a load and wherein a drive mechanism is deenergized whenever the sensor detects the presence of such object while the load is being driven in a given direction and then is energized to drive the load in the opposite direction.
A typical example of such system is found in drive systems which are used in driving side window pane and roof panel of an automobile. An exemplary side window pane drive system is disclosed in U.S. Pat. No. 4,394,605 issued to Terazawa. This patent discloses an embodiment in FIG. 2 thereof comprising an up/down momentary command switch which provides a command by a one-touch action, a combination of up and down self-hold circuits which maintan the energization of an electric motor as indicated by the command from the switch, a resistor which detects a load current through the electric motor which is used to drive a side window pane of an automobile up and down, a pair of comparators for comparing the current flow through the resistor against a reference value, a pressure sensitive switch for detecting the presence of any object which may be interposed between an edge of the pane and an associated window frame during the upward movement of the side window pane, a reset circuit for resetting the both self-hold circuits to deenergize the motor whenever the load current goes outside a given range, and a circuit for activating the down self-hold circuit in response to the detection of an object by the pressure sensitive switch. In that patent, in particular, in an embodiment shown in FIG. 2 thereof, the temporary closure of the up command switch with an up contact allows the up self-hold circuit to be activated to energize the motor for forward rotation, thereby causing the side window pane to move upward vertically. If an object exists between a window frame and the window pane, and the pane moves into abutment against the object, the pressure sensitive switch is closed to activate the down self-hold circuit. Thereupon, an equal voltage is applied to the opposite terminals of the motor, which therefore comes to a stop. Thereupon, the load current reduces below the lower limit of the given range, whereby one of the comparators is reset, thus resetting both the up and the down self-hold circuit. Accordingly, it will be seen that whenever the pressure sensitive switch has detected an object, the motor comes to a stop and the system returns to its standby condition where it waits for another operation of the up/down momentary switch. However, it is desirable in such system that the side window pane be automatically driven downward, rather than stopping its movement whenever the pressure sensitive switch has detected an object.
Japanese Patent Publication No. 44,445/1973 discloses another side window drive system in which if a pressure sensitive switch becomes closed during the time an electric motor is energized for forward rotation to move a side window pane upward, the motor is energized for reverse rotation to move the window pane downward. However, in this system, the motor is energized for reverse rotation only during the time the pressure sensitive switch remains closed. Hence, as soon as the window pane is lowered slightly in response to the energization of the motor for reverse rotation, the pressure sensitive switch becomes open to interrupt such energization. The motor is then energized for forward rotation which is previously established, again causing the pane to press against the object. It will therefore be seen that the pane repeats its movement up and down as long as the object exists, presenting a difficulty in removing the object which is located between a window frame and the pane. In such system, it is desirable that the pane be automatically driven for downward movement, rather than repeating its upward and downward movement, whenever the presence of the object is detected.